Tuesday, September 13, 2011

My Favorite Reaction

Chemical and Engineering News is hosting a Blog Carnival - a gathering of blog postings around a common theme, which in this case is "My Favorite Reaction". In my mind, you can't have a Carnival without a lion (or a lion tamer),

and so using my best barker's voice I invite you "to step right up and prepare to see the Greatest Show on Earth..."

In most cases, free-radical polymerization can be compared to a hungry lion on the Serengeti, randomly grabbing wildebeests and zebra as they scurry by. First a wildebeest, then a zebra, then maybe two or four wildebeests, then back to a zebra again, all in no particular order until the last of the herd is gone. This is what occurs in free-radical polymerization when more than one monomer is present - the result is a statistical distribution of the different comonomers. Enter the lion tamer, who is able to control the lion's appetite so it carefully alternates back and forth between the wildebeests and the zebras. The lion tamer goes by the name "thiol-ene polymerization". In this reaction, the growing chain clearly and forcibly alternates back and forth between the two monomers present, the thiol and the ene of whatever sort they may be, resulting in a perfectly formed alternating copolymer --ABABABABA--. The two main reaction steps are:

Were the formation of an alternating copolymer the only advantage of thiol-ene polymers, the lion would have been tamed, but left as a small amusement in the Carnival's sideshow and this post would never have been written. In fact the advantages of thiol-ene chemistry extend far beyond the regularity of the polymerization. Besides being a solvent-free/water-free reaction, thiol-ene systems can react fast, shockingly fast. I've developed floor coatings that under UV light have converted monomers into a hard coating, suitable for walking on, in one-tenth of a second using just a lamp plugged into a 120 VAC outlet, not a souped-up unit drawing enough electricity to power half of Tokyo.

But even better yet is that these systems do not suffer from the oxygen inhibition that commonly occurs in UV cured-acrylates. The hydrogen of the thiol group is labile enough that when oxygen adds to the chain to form peroxy radicals, the radicals are still able to abstract that hydrogen, thereby creating a new thiyl radical allowing the polymerization to continue. Acrylate polymerization comes to a standstill. In summary, you have: